1. Field
Exemplary embodiments of the present invention relate to a nonvolatile memory device, a method for fabricating the same, and a method for driving the same, and more particularly, to a resistive random access memory (ReRAM) having a threshold switch element provided in a bidirectional resistance change layer, a method for fabricating the same, and a method for driving the same.
2. Description of the Related Art
Existing nonvolatile memory devices are represented by a flash memory. Recently, research has been actively conducted on next-generation memories having new materials and structures, in order to replace the existing nonvolatile memory devices. As one of the next-generation memories, a resistive random access memory (ReRAM) may have features of a low production cost, a simple fabrication process, low power consumption, and a high read/write speed. Accordingly, much attention has been paid to the ReRAM. Furthermore, since the ReRAM may employ a cross-point structure, the ReRAM may satisfy the recent demand for a high-capacity memory device. In the cross-point structure, however, a leakage current may flow through adjacent cells. In order to solve the concerns, research is being conducted on a method for connecting a switch element such as a transistor or diode to ReRAM.
For example, in the cross-point structure, a changeable resistance material is deposited at an intersection between a word line and a bit line and connected to a transistor or diode. However, when the transistor is applied to ReRAM, the size of the ReRAM increases. The diode may not be applied to ReRAM using a bipolar voltage. Therefore, it may be difficult to maximize the features of the ReRAM.
Recently, a nonvolatile memory device including a resistor having a threshold switch characteristic and a memory array including the same (U.S. Pat. No. 7,935,952) have been disclosed, in order to solve a leakage current problem in the ReRAM and enhance the advantage of the ReRAM.
In the above-described patent document, a threshold switching layer having a threshold switch characteristic and a resistance change layer are connected in series. The above-described patent document discloses a graph showing that the threshold switching layer is connected in series to a unipolar memory where a set operation or a reset operation occurs in one voltage state.
FIGS. 1A and 1B are current-voltage graphs illustrating electrical characteristics of the resistance change layer and the threshold switching layer in the conventional nonvolatile memory device.
Referring to FIG. 1A, the resistance change layer has a high resistance state (HRS) or a low resistance state (LRS) when a voltage is not applied. An operation for changing resistance state of the resistance change layer from the HRS to the LRS is referred to as a set operation. A voltage to cause the set operation is referred to as a set voltage Vset. On the other hand, an operation for changing resistance state of the resistance change layer from the LRS to the HRS is referred to as a reset operation. A voltage to cause the reset operation is referred to as a reset voltage Vreset. In the resistance change layer having a bidirectional characteristic, the set operation and the reset operation occur at different voltage polarities. For example, in FIGS. 1A and 1B, a positive voltage difference cause the set operation; and a negative voltage difference causes the reset operation.
Referring to FIG. 1B, the threshold switching layer can be in a low resistance state Rmetal, which is similar to that of a metal, or a high resistance state Rinsulator which is similar to that of a nonconductor. The threshold switching layer may have different characteristic curve from the resistance change layer because the threshold switching layer has the characteristic of a nonconductor in a state where a voltage is not applied. Furthermore, when a given voltage is applied to the threshold switching layer, an amount of currents rapidly increases and a resistance rapidly decreases. At this time, the applied voltage is referred to as a threshold voltage Vth. On the other hand, when the voltage level applied to the threshold switching layer decreases, an amount of currents rapidly decreases and a resistance rapidly increases. At this time, the voltage is referred to as a sustain voltage Vh. Furthermore, the threshold switching layer has a symmetrical electrical characteristic depending on the polarity of a voltage applied across the threshold switching layer. Therefore, threshold voltages Vth+ and Vth− based on an applied voltage difference have the same magnitude. Sustain voltages Vh+ and Vh− also have the same magnitude.
FIG. 2 is a conceptual diagram for explaining a read operation of the conventional ReRAM.
Referring to FIG. 2, the ReRAM includes a plurality of word lines and a plurality of bit lines. A plurality of memory cells are arranged at the respective intersections between the word lines and the bit lines. Each of the memory cells has a resistance change layer and a threshold switching layer.
In order to perform a read operation, a specific voltage level is applied to sense a difference between resistive states of the resistance change layer. In order to perform a read operation on a unit cell in the ReRAM array having a cross-point structure, a voltage of −Vread/2 is applied to a bit line, and a voltage of Vread/2 is applied to a word line. Therefore, a voltage difference of Vread for a read operation is applied between the bit line and the word line coupled to a selected cell. In this case, however, a voltage difference of Vread/2 is applied between the bit line and the word line coupled to an unselected cell.
FIGS. 3A and 3B are graphs illustrating the resistance states of the conventional ReRAM.
FIG. 3A illustrates resistive states of selected unselected cells, during a read operation, which includes a resistance change layer and a threshold switching layer. When only the resistance change layer is included and the threshold switching layer is not embedded, a voltage of Vread/2 is applied to both the selected cell and the unselected cell adjacent to the selected cell. When the unselected cell is in the LRS during the read operation, a current path may be formed because of the voltage of Vread/2. Therefore, when the size of the cell in the cross-point structure increases, the amount of current flown with respect to the applied voltage of Vread/2 increases. In this case, it may be impossible to correctly recognize the amount of current flowing in the selected cell.
Referring to FIG. 3B, when the threshold switching layer is embedded, a current flow may not be formed because of the HRS maintained when a voltage is not applied. Accordingly, it is possible to discriminate the selected cell from the unselected cell during the read operation. A method for minimizing an error during the read operation has been researched to limit the amount of current flowing through the unselected cell by the threshold switching layer.
However, the above-described conventional ReRAM has a sequentially stacked structure of the threshold switching layer and the resistance change layer. In particular, when different layers are deposited at a high temperature, an unexpected characteristic change may occur at the interface or contact area between the different layers. Furthermore, if defects are concentrated on the interface or the contact area, it may be difficult to practically implement a semiconductor memory device, such as ReRAM, based on a theoretical current-voltage characteristic.